Pulse modulation system



Aug; 20, 1946. E. LABIN PULSE MODULATION SYSTEM Filed April l. 1941 4Sheets-Sheet 1 ATTORNEY Aug. 20, 1946. E. LABIN PULSE uonULATIo SYSTEMFiled April 1, 1941 4 Sheets-Sheet 2 INVENTOR .NE Y

ATTORNEY l Allg. 20, 1946. E, LABlN 2,406,019

PULSE IODLATION SYSTEM Filed April 1, 1941 4 Sheets-Sheet 3 FIGA;

lNVENTOR ATTCRN l Aug. 20, 1,946. E. LABIN PULSE MODLATION SYSTEM 4Sheets-Sheet 4 vFiled April 1, 1941 6 M a MW w amava/.Aw

wn-calml l reared' 20 1946 amig PULSE MODULATON SYSTEM application Aprili, i941, serial 10,386,282

(ci. 25o-6) i2 claims. l

This invention relates to improvements in pulse modulation systems.

It is a primary objective of the invention to provide an improved pulsemodulating system.

Another object is to provide more interferencefree system of radiocommunication.

A further object is to provide an improved system of radio transmissionwhich may function despite deliberate attempts to jam the transmissionfrequency.

It is also an object to provide an improved and simplified pulsemodulation reception system.

Still another object is to provide an improved system of secret radiotransmission.

A further object is to provide an improved form of pulse modulationsystem in which a. single, constant-width impulse may suiilce forindication of intelligence.

It is also an object to provide a pulse modulation system adaptable torelatively simple receiver means.

Another object resides in provision of a pulse modulation system inwhich there is no phase modulation of, the impulses.

Other objects and various further features of novelty and invention willhereinafter be pointed out or will occur to those skilled in the artfrom a reading of the following specification in con, junction with thedrawings included herewith. In said drawings Fig. l is a graphicalshowing of the effect of signal treatment in accordance with features ofmy invention;

Fig. 2 is a block diagram of a pulse transmitter according to myinvention;

Fig. 3 is a block diagram of a pulse receiver incorporating features o-fmy invention;

Fig. 4 is a schematic circuit diagram of a portion of the transmitter ofFig. 2; and

Fig. 5 is a graphical showing of successive changes in wave shape inportions of the circuit of Fig. 4.

Pulse modulation systems are known in which the amplitude of the pulseis always maintained constant and the duration of the pulse vmade tovary in accordance with speech or other modulating signals. It is alsoknown that the whole pulse-so modulated need not be transmitted, butrather two pulses, each marking one determinant end of the modulatedpulse. This latter method has advantages over the former, in that stillgreater-signal-to-noise ratios may be obtained, but has the disadvantagethat a signal so transmitted requires just as much of a time-interval(i. e., from the beginning to the end of the modulated pulse) to beintelligible as in the former case. It might also be observed thatneither of the above-indicated systems is free of phase modulation, aswill later be clear.

In accordance with features of my improved form of pulse modulationsystem only one extremely short pulse is needed to identify a. signal,no matter what the degree of modulation. This improved methodcontemplates the time-displacement of a single pulse one side or theother about a given norm in accordance with the degree of modulation;hereinafter alternately referred to as time-modulation. Since themaximum time-displacement of the impulses is preferably small incomparison with the time-interval between pulses. a large part of thetime is not used for transmission; and suitable receiving apparatus mayinclude a sharply selective circuit tuned to the pulse frequency andblocking means energized by said selective circuit for excludingsubstantially all noise but the displaced impulses. Appropriatedisplacement responsive means may then detect out the audio` signal in aknown man- I consider it preferable to a more ready understanding of myinvention rst to indicate generally how I treat signals in accordancewith features of this invention, and then to describe possible means forgiving such treatment. Accordingly, vreference will first be made toFig. 1 which represents graphically the general treatment of pulses inaccordance with features of my modulating system.

The pulses modulated as indicated correspond to a spectrum offrequencies with bands located near the harmonics of the pulserepetition frequency. Each harmonic may be modulated in amplitude bytime modulation of the pulses. If any particular harmonic of thetransmitted energy be selected for detection or demodulation, theoriginal signal may be restored by 'relatively simple means forming a,feature of this invention.

In order to avoid combined amplitude and phase modulation of harmonicsand increase the percentage of amplitude modulation of harmonics, Ipropose to displace alternate impulses symmetrically and in opposedsenses about their normal unmodulated position. In other words, theimpulses are displaced to new, modulated positions which are in timesuccessively before and after` the normal instants at which unmodulatedimpulses would occur. In such a modulation system the impulses may beconsidered as made up I, 3, kare shown schematically with respect totime and as being time-modulated in a sense opposed to that of themodulation of even impulses 2, 4, Ii.` It wil1 be clear that if torepresents the maximum amplitude of time displacement for the Yimpulses, one set of impulses I, 3, 5 etc. will be displaced :2te withrespect tothe other (2, 4, 6 etc.) for maximum modulation, dependingupon the magnitude of modulation.

As stated above, the series of pulses shown in Fig, 1 may be consideredas formed of two identical series, each having a period T, which serieswe shall designate as P1 (for pulses I, 3, 5 and P2 (for pulses 2, 4, 6respectively.

By appropriate choice of time origin the center of the rst pulse I ofseries P1 may occurat time AP: Em(- 01, and the amplitude A of pulse Imay be described as- AP1=ZMAW cos mou-61) in which Ap2=2,.,.-cos mw(t02)(2) Where 62 represents the time at which the center of impulse 2 occurswith respect to the time origin.

By combining expressions (1) and (2) it will be clear that the combinedeiect of both trains of impulses will be described by- If `the timeorigin has been appropriately selected, a simplied solution may result.Assuming, then, a time-displacement of both series P1 and P2 inopposite. directions by a time 0, We may arbitrarily describe 61 and 62as- T 01 b- 9 Hand in which 2b arbitrarily represents the distancebetween the axes of modulation of the ilrst two pulses after the timeorigin, and from which as will be clear. After substituting Equations 4and 5 in expression (3), we obtain Now, if we assume that thetime-displacement Such an assumption will permit the followingsimplifications for our purposes:

cos mw0==11 and tall mw9=mw0 Wherefrom an appropriate expression for Apbecomes Ap being the sum of amplitudes at frequencies "/r each of whichis amplitude modulated bythe signal 0, as will be clear.

Now, assuming the simplified case of 0 sinusoidally varying with aperiod a, We may write l 0=K.o. Sin (at-Ho) v(10) Upon examination ofexpression (9), it will at once be observed that the depth of'modulation for the mth harmonic is mao. tan mwb=K.e. tan mob It will berecalled that K is always equal to or y less than 1, and that e is muchless than 1. Therefore. in order to obtain large orders of modulationKe. `tan mwb should approximate unity, or tan mwb should equal will beclear, would give rise to the undesirable 1 effect of doubling theimpulse frequency. In order, then, to avoid a doubling of the period-T,

characteristic of each seriesl of impulses, we may define b as T i b=Eto(11) in which case, for m odd,

tan mwb= =1 tan e e and cos mwb=sin e=e and, for m even,

and

-Y cos mob-:cos e=1 The general term of series (3) may now be written intwo forms, depending on whether m is odd or even; thus, for m odd2Am.e[1-K sin (at-l-qnl cos met From expressions (12) and (13) it willbe noted that the presence of e in the amplitude term will make for oddharmonics of relatively small amplitude, while even harmonics, not beingassociated with the factor e, will he relatively large. On the otherhand, odd harmonics are highly modulated, whereas there is hardly anymodulation of even harmonics.

It will further be noted from these same expressions that, inasmuch asai and 02 vary in opposite directions, there are no phase modulationterms, and that, therefore, all modulation is in amplitude. That suchrelative variation of 6i and 02 is responsible for'elimination of phasemodulation may be seen if we consider one series of impulses asunmodulated while modulating the other. In such an assumed case, for modd, we would obtain, instead, of expression (12) ZAWeIII-- sin (at-H]cos mw(tg) (14) Y In this case it will be noted that depth of modulationis necessarily only one half that of the preferred case of expression(l2), and that phase modulation is present. It may be observed, however,in this connection, that the phase modulation present in the case ofexpression (14) is not very harmful to speech transmission; but, ofcourse, for utmost fidelity purposes, it would be consideredundesirable.

Returning again to the preferred system in which both series of impulsesare modulated in opposite directions, let us consider reception anddetection of signal impulses. Since each odd harmonic, as will be clearfrom expression (12), is purely amplitude-modulated, a circuit tuned tosome such odd harmonic of l/T would permit the amplitude modulated termto be removed and thus result in detection of the signal. Furtherutilizing received energy in accordance with features of the invention,any Veven harmonic term, which as we have seen from expression (13) maybe very large, may be employed, after suitable shaping. to obtain agenerally squareshaped blocking signal for blocking out reception ofenergy for substantially the interval .between pulses. By making theeven harmonic circuit as selective as possible, it will be clear thatparasitic disturbances may be reduced to a minimum, and thatsubstantially only the signals themselves will be admitted to thedetection circuits.

A further desirable feature of my invention may be seen in its use asanimproved secrecy system. Since the steady component existing in thesignal is modulated but very slightly, ordinary detection methods willonly be able to detect a small unintelligible disturbance, and commoncircuit and other noise will probably be large enough compared with suchvery slight modulation that the signal may be completely unintel-yligible, as will be clear.

This brings us to a consideration of the signalto-noise ratio andrelated factors. If we assume that the receiver is blocked continuouslyexcept during the pulse and modulation interval 2me-d, d rheling thelength of the pulse, the probability of interference for equalamplitudes of desired and interfering signals may be expressed assumingan interfering signal substantially uniform with respect to time. Ofcourseffor best reception, free as possible from interference.

.should be made small as possible; and in the case of It will be seenthat, for any given value YofV e, p is inversely proportional to m, andthat, therefore the higher the harmonic chosen for detection, thegreater will be freedom from interference. Since e may be selected asdesired, it will also be appreciated that to should be as small aspossible, both in order to obtain less probability of interference andto see that modulation (see expression (7)) follows a substantiallylinear4 rather than a tangential law. In review then, as high a harmonicas possible should so be chosen that, when considered in view of thesmallest possible to and d, the term e is not so large that linearmodulation is impaired. In any case, the final value chosen will be acompromise between the frequency band available and freedom frominterference.

In the drawings I show a preferred possible circuit for obtaining theabove-indicated desired results in accordance withfeatures of theinvention. In Fig. 2, which shows a transmitter in block diagram form,impulse energy may be the transmitter diagram.

Fig. 4 shows a possible circuit for converter 2| schematically and insome detail. In a preferred form the circuit is designed to set up thetwo series of pulses l, 3, 5 and 2, d, 6 so that, when speech or othersignals are applied, time-modulation of one set of pulses will beopposed to time-modulation of the other. The circuit of Fig. 4 may morereadily be understood when viewed jointly with the wave forms of Fig. 5.First, we shall consider the case of no modulation, tracing wavetreatment throughout the circuit of Fig. 4, as depicted, in anexaggerated `manner for purposes of clarity, by waves a is. so that theimpulses later to be superimposed occur at instants of timecorresponding to positive and negative maxima of the input voltage. Acircuit for eiecting the desired phase displacement may. for example,include series capacitance means Ca and shunt resistance means Re, asshown.` 'I'he input sinusoidal voltage, so shifted in phase may then besubstantially squared on by well-known limiter means including anoverexcited amplifier tube V1 and appropriate circuit elements. Outputenergy from tube V1 may be taken from across a potentiometer 25 and ispreferably divided for application to tubes V2 and V3 in push-pull. Theoutputs of tubes V2 and Va may be connected in parallel as shown so thata unidirectional output signal may be obtained. For purposes hereinafterto be indicated the t'ap of potentiometer 25 is so adjusted that thepush-pull control potentials applied to tubes V2 and V3 are unbalanced;that is, one is substantiallygreater than the other. As a result of thelimiting eiiect, adjustment of `potentiometer 25, and the paralleloutput connection ofthe tubes V2 and V3, an output wave form somewhatconforming to Fig'. `5b is obtained.

At this Ystage f the sinusoidal input signal treatment impulse energy(shown in Fig. c to be substantially in quadrature with the sinusoi'clalvoltage, that is, characterized by impulses` occurring as the sinusoidalinput passes through zero) may be superimposed upon the wave form ofFig. 5b by appropriate grid control means 26, 21 shown associated withtubes^V2 and Va', respectively. The resultant wave so obtained will beof the general form shown in Fig. 5d as will be clear.

In accordance with features of my invention, I next apply the Wave ofFig. 5d to a multivibrator circuit of known form including tubes V4 andV5. These multivibrator tubes are preferably so excited that an impulse,say 21 (Fig. 5d) of a certain magnitude will energize one of the tubesV4 and V5, and a drop below a certain exciting potential, say 28 (Fig.5e), will cause operationof the other multivibrator tube. The circuit ofthese tubes (V4 and V5) preferably includes appropriate time-constantdecay circuit means for deriving an output from the multivibrator of thegeneral form shown in Fig. 5e. The multivibratorcrcuit including tubesV4 and V5 is one capable of controlling the instant of transition fromone condition to another, and operates spontaneously without externalcontrol to return to the rst condition. The multi-vibrator action isattained by virtue of a conventional forward coupling from tube V4 totube V5 over a resistance capacitance arrangement R7, C1, R1, togetherwith a back coupling from the output of tube V5 to the input of the tubeV4 by virtue of a common cathode resistor R. The forward couplingthrough R7, C1, R1 is like an ordinary interstage coupling except thatthe values of condenser C1 and resistance R1'are such that condenser Cimay be charged to a substantial value in a few microseconds.-

In considering operation of the multi-vibrator circuit, assume as aninitial condition an instant or so before occurrence of impulse 21 (Fig.5d), when tube V4 is carrying relatively high plate current; thusmakingits plate far less positive than the potential of plate supply. Thisreduction in the plate potential of tube V4 has the effect of applyinga'negative potential on the control grid of tube V5 after a delay,instituted by condenser C1 and resistor R1, suflicient to allow thenegative potential to be substantially attenuated and permit tube V5 todraw grid current. Thisy is the condition of tubes V4 and Vs when pulse21 appears. Application of pulse 21 to tube V4 momentarily drivesthatidischarge device below cut-oi, thus permittingv a sharp increase inplate voltage. This relatively positive potential on the plateistransmitted through condenser C, to render tube V5 conducting.Thereupon, because' of the conducting state of tube Vt, a resultant dropin cathode resistor R shifts cathodes of both multi-vibrator tubespositively. This shift has the effect of making the control grid of tubeV4 so negative with respect to the cathode that the cut-off conditionset up by the momentary impulse is maintained a little longer. v

The cut-oi state of tube V4 will in every instance be maintained for aperiod of time depending upon the magnitude of negative impulse applied.For example, pulse 21 being more negative than pulse 30 will retain tubeV4 in the cutoi state longer than pulse 30 will. This will be clearlyseen from the fact that condenser C1 is variously charged depending uponthe magnitude of the positive pulse output of tube V4 when the lattersconductivity is first cut off. This positive potential in the form of acharge -on condenser Ci will continue to be applied to the control gridof tube V5 (thus keeping tube Va conducting) until the leakage effect ofresistance Riso reduces the magnitude of the charge that the outputcurrent of tube V5 is insuiiicient to maintain the drop in cathoderesistor R necessary to keep tube V4 below cut-oil. When the drop inresistor R gets that low, then, tube V4 will again conduct and tube Vswill be non-conducting. This is the point at which the operation of themulti-vibrator was assumed to commence, and so a complete cycle ofmulti-vibrator action has been described.

It has just been pointed out that a longer time will elapse before decayto a predetermined trip level 28 for larger applied pulse magnitudesthan for lesser applied pulse magnitudes. An attempt has been made Atoshow this phenomenon in the curve of Fig. 5e. It will be obserbed thatpulse 21 and pulse 38 of Fig. .5d represent respectively pulses one ofgreater magnitude than the other. Pulse 21, being relatively larger,causes a relatively large output voltage 29, with the result that decayfrom this voltage to the level 28 takes a relatively long time t'. Pulse30, being xsomewhat smaller in magnitude than 21, consequently causes alesser output voltage 3l, with the result that decay from yvoltage 3| tolevel 28 takes a correspondingly smaller time interval t than t in thecase of pulse 21.

It will now be clear that the respective widths t and t" each recurregularly with a periodicity T. In accordance with features of my invention I employ the relative position of the instant at which the decayingvoltages reach level 28, that is, when the drop across cathode resistorR due to conduction of tube V5 decreases to such a magnitude that thecontrol grid of tube V4 is no longer effectively biased beyond cut-oil?,to determine the two series of impulses I, 3, 5 and 2, 4, 6 of period T.

To obtain the two series of pulses from the curve of Fig. 5e I prefer toemploy a Well-known resistance-capacitance derivative circuit, whichmayinclude capacitance C2, resistance R2 and an over-biased tube Vs, inthe output of the multi-vibrator. Output from the derivative circuit, asobtained across R3 may be of the form of Fig. 5f, as will be clear. Itwill further be clear that peaks 32, 33, 34, etc. so obtained representthe desired. appropriately displaced two series of impulses. Since theother peaks 35, 36, 31, etc. obtained from the derivative circuit areall regularly spaced with period T/2 andare independent of themagnitudes of pulses 2l, 3d, they are of no use to a transmitter inaccordance with my invention, and may therefore be suppressed bywell-known means (not shown), as I have indicated schematically in Fig.g. The unmoduiated signal of Fig. 5g is now fully prepared for itstransmission by any desired means.

The case of unmodulated pulses has first been taken for discussionbecause it is obviously simpler than when the impulses are modulated.Figs. 5h through 5l illustrate this latter more complicated case for anassumed sinusoidal modulating potential, shown in Fg. 5h. It will beclear that the voltage of Fig. 5h may represent speech or otherintelligence to be transmitted..

In a preferred form this speech may be applied to modulate the pulsesignals at the push-pull rectider stage V2, V3 of the converter of Fig.4. According to this embodiment, balanced, that is equal, speech signalsare applied by means of an input transformer 33 to appropriate controlgrids of rectiers Ve and Va.

The edect of such application of speech signais may be to vary the curveof Fig. 5d as shown in Fig. 5i, that is, to make pulses corresponding to2l and 3d of either greater-or lesser relative magnitude in accordancewith the modulation. Thus. pulse 39, being at an unmodulated point, isunmodulated, and hence of the same magnitude as pulse 217. However,pulse 40, being displaced by the rising portion of the modulatingpotential, is of lesser magnitude than pulse 3i). Recalling thediscussion in connection with Fig. 5e, it will be seen that pulse 39will cause a multivibrator output signal of duration t'; Whereas pulsedd, being of lesser magnitude than pulse 3d, will cause a multivibratoroutput signal of still shorter duration (tm) than t", as will be clear.

The next succeeding impulse il of series 39, el, d2, which maycorrespond to series i, 3, 5 of Fig. l, has been so modulated that itspeak is of considerably greater magnitude than those of 2t or 39.Consequently, voltage decay from the multivibrator output voltagecorresponding to pulse di may be of proportionally greater duration tm'than t', for pulses 2l and 39. Considering this phenomenon as to itseffect on the displaced peaks of Fig. 5k, it will be apparent that, asthe modulating signal is, say, increasing, the pulses (3W di) of oneseries (39, di, d2)

are time-modulated or displaced in one direction (i. e.. retarded intime) from their 'unmodulated relative positions, and the pulses (saytii) of the other series (fili, 43, M) are time-modulated or displacedin the opposite sense (i. e., advanced in time) from their unmodulatedrelative positions. When prepared for transmission treatment, themodulated pulses, corresponding to a modulating signal of Fig. 5h, maythen resemble the curve of Fig. 5e, as will be clear.

In the circuit of Fig. 4 it will be appreciated that many adjustmentsmay be made to vary the relative displacement of the two series ofpulses, width of pulses, modulation depth, etc. etc. For instance, bychanging the value of resistor R,- or varying the bias on tube V4, thewidth of pulse in Figs. 5e or 5i may be controlled,in that amplication,thus controlled, willoccasion diiferent pulse widths t or t", etc. Thus,an increase of resistance R, or increase in the negative bias of tubeV4, will displace all pulses by increaslngthe retardation or decay time.Conversely; a decrease in R, or decrease in bias, will result in anopposite displacement eect. Relative displacement of the two series ofpulses, one with respect to the other, may be varied by adjustment ofthe ground tap on potentiometer 25, which, as we have seen, controls therelative amplitudes of adjacent lobes of the curve of Fie. 5b. v

While a form of converter circuit 'shown has been described inparticular detail, it is to be understood, of course, that suchdescription is merely by way of example to illustrate how the desiredform of impulse treatment may be effected.

A possible and preferred form of receiver for detecting impulses,transmitted as outlined above, is shown schematically in Fig. 3. Impulseenergy received by antenna t5 may rst be treated in a customary mannerin radio and intermediate frequency amplier stages,f shown generally asa block B6. Thereupon it may be detectedl and some disturbing effectsremoved by suitable limiter and detector means dl. At this point, inaccordance with a feature of my invention I provide means, responsive tothe output of detector t?, for blocking out reception of any signalssubstantially for the period between successive impulses. Such blockingmeans preferably includes a sharp selective circuit 68, tuned twice tothe pulse frequency (i. e., of period T) and a square-wave generator t9controlled thereby. By suitable adjustment means (not shown) thesquare-wave output of generatoril may be made to produce a blockingsignal occurring only when no impulses are due for reception. 'Ihisblocking signal when applied to the earlier stages 46 or l of thereceiver may effectively cut out substantially any and all parasiticdisturbances which would otherwise interfere with reception of theimpulses.

A relatively simple blocking signal of uniformly recurring duration maybe derived by well-known means utilizing, say, the second harmonic ofthe impulse period T. Such a signal could, for example, be obtained byhalf-wave rectification of the second harmonic and adjusting generator49 to give as large a constant-width blocking signal as possible withoutinterfering with impulse reception. Suitable oparatus for obtaining asignal of the indicated nature having a desired l `width has beendisclosed in the copending application of H. G. Busignies, Ser. No.380,186, filed February 24, 1941, and entitled "Radioelectric impulsesystems." Such a blocking signal is shown schematically by thedash-dot-dot line 50 in Fig. l, and is designated as a simple blockingsignal. It will be observed, however, that such a blocking signal is noteffective completely to block out reception between pulses in that oneseries of pulses, as we have seen, is other than exactly in phaseopposition to the other series of impulses, and, furthermore, modulationof one series is opposite in sense to that of the other. What is needed,therefore. for much greater blocking efciency, is some sort of complexsignal which will block reception for both the short interval of timebetween, say, pulses i A possible arrangement for this odd propor`tioning of blocking signals could include apparatus similar to thatoutlined above in connection with the simple blocking signal (for theone particular odd harmonic.

. y 11 shorter intervals) and an additional superimposed blocking signalsuitably phased with respect to the first-mentioned blocking signal inaccordance with teachings in the above mentioned H. G. Busigniesapplication and recurring with a period T for substantially blocking outthe longer intervals. It will be clear that known wave-shaping methodsmay bel employed for utilizing preferably the second harmonic to obtainthe additional superimposed, as well as the firstindicatedblocking'signal. The complex blocking signal so obtained has beendesignated as such in Fig. 1 and is shown as a dotted line.

After the above-indicated treatment o f received signals, it will beseen that output tothe rest of the receiver vial line 52 will includeonly the pulses themselves with a very slight amount of adjacentparasitic noise, which may completely have been eliminated in thelimiter circuit 41. It will be recalled from the above discussion oftheory involved that any odd harmonic in line 52 will includeamplitude-modulated pulses; see expression (12). Thusfall that now isneeded is an appropriate band-pass filter 53 for selecting the desiredodd harmonic, the passband being, of course, wide enough to accommodatethe modulating frequencies either side of the harmonic selected.Thereafter, simple detector 54, audio amplier 55, and speaker means 56may restore the original input signals with theoretically perfectildelty. as will be appreciated.

It might here be observed that theoretically any amplitude-modulationreceiving set ought to be able to listen-in on pulse-modulated sig.nals` in accordance with features of this invention. Practically,however, it seems unlikely that such will be possible. for it would benecessary to have a set including an extremely stable and critical localoscillator for receiving only Furthermore, an ordinary set would not beprovided with my novel blocking means, so that the averagesignal-tonoise ratio might, and would in all probability, be so great'asnot torpermit reception at all. Thus, it will be seen that transmissionand appropriate reception in accordance with my invention may even takeplace without suspicion by anyone that any such communication is takingplace. N

In accordance with ,still another feature of the invention, still bettersignal-to-noise ratios may be obtained. I propose to utilize theinterval between blocking impulses (in the receiver), which interval isrelatively short compared to the time between pulses, momentarily toincrease the gain,

say, of amplifier 55. Although the means for so improving reception hasnot been shown, it will be appreciated that it may include suitablewave-shaping means for applying a momentary abnormally high voltage tothe anode or, say, an

accelerating electrode in an amplifier tube of circuit 55. Whilesustained application of such a voltage might ordinarily harm this tube,a momentary application thereof conceivably will not. At the same time,it will be clear that amplification and efliciency of the tube may bematerially increased for this, the desired and useful instant of time.

It will be seen that I have provided a relatively simple radiotransmission apparatus of greatly improved efllciency and having manyother desirable features. While the invention has been described inparticular detail and preferred forms illustrated, it is, of course, tobe selecting frequencies in the neighborhood of an odd harmonic of the'impulse frequency, whereby it is possible to demodulate by ordinaryampli,

tude detection of an odd harmonic of the impulse frequency.

2. A radio communication system including a transmitter and receiver,said transmitter including impulse generator means for generating twoseries of impulses of the same impulse frequency, and means fortime-modulating one of said series of impulses with respect to th'eother of said series, said receiver including filter means passingsubstantially only a band of frequencies immediately about an oddharmonic of the impulse frequency of one of said series, andamplitude-detection means.

3. A receiver for an impulse-modulation communication system, saidreceiver'including lter means passing a band of frequencies including anoddv harmonic of the frequency of the received impulses,amplitude-detecting means, 'and blocking means responsive to an evenharmonic of said frequency of the received impulses, said blocking meansincluding a generator responsive to said even harmonic for supplyingenergy to cut out reception of signals during substantially the periodbetween received impulses.

4. In a receiver for receiving and detecting time-modulated impulsevenergy, blocking means for suppressing reception during the-intervalbetween impulses of the received impulse energy, said blocking meansincluding a sharp selective circuit attuned to an even harmonic of thefrequency of said received impulses, wave-shaping means for generating agenerally square-shaped signal to occur substantially duringthe'interval between said impulses, and means responsive to the outputof said last-mentioned means for suppressing effective operation of saidreceiver in accordance with said generally square-shaped signal.

5. An impulse time-modulation transmitter according to claim 11, inwhich the impulses generated by said impulse generator are relativelyshort with respect to the interval between successive impulses, and inwhich, for the same degree of modulation, time retardation of impulsesof one of said series is of substantially the same magnitude as the timeadvance of impulses of the other of said series.

6. In an impulse time-modulating transmitter, an impulse generator forsupplying a series of regularly spaced impulses, circuit meanssynchronously related to said impulses for supplying a full-waverectified alternating wave of a frequency one-half the recurrencefrequency of said impulses and having oddv maxima greater than evenmaxima, means for superposing said impulses and said rectifiedalternating wave, multivibrator means responsive to such superposed 13multivibrator means for deriving impulse peaks corresponding toalternative action of said multivibrator means.

7. A transmitter according to claim 6, in which additional circuit meansresponsive to modulating energy is provided for modulating the magnitudeof successive maxima of said Wave.

munication system, said receiver including filter means passing a bandof frequencies including a harmonic of the frequency of the receivedimpulses, amplitude-detection means, and blocking means responsive to aharmonic of said frequency of the received impulses, said blocking meansincluding a generator responsive to said last-mentioned harmonic forsupplying energy to cut out reception of signals during substantiallythe period between received impulses.

l0. A receiver for detecting time modulated impulse energy, odd impulsesof said energy recurring with a given periodicity, said receivercomprising filter1 means passing substantially only a band offrequencies immediately about an odd harmonic of said periodicity andamplitude detection means.

11. In an impulse time-modulation transmitter, an impulse generator forgenerating a rst series of regularly spaced impulses and another seriesof regularly spaced impulses of the same frequency of recurrence as saidfirst mentioned series, modulating means for time-modulating impulses ofsaid first mentioned series of im# pulses in one sense and fortime-modulating impulses of said other series in an opposed sense, andmeans displacing impulses of said rst series in time with respect tothose of said other series, said displacing means being eiective todisplace said iirst and said other series respectively to an extent thatan impulse of said first series always precedes an impulse of said otherseries with an interval of time greater than that by which an impulse ofsaid rst series follows animpulse of said other series, wherebysucceeding impulses alternately determine an interval greater thanone-half the periodic recurrence of impulses of one of said series and asucceeding interval less than said one-half.

EMILE LABIN.

